As a near-infrared sensor having sensitivity in the near-infrared wavelength range of 1.7 μm to 2.6 μm, one having such a structure has been known in that a p-type semiconductor layer is formed by diffusing zinc in an epitaxial wafer having an InzGa1-zAs layer (hereinafter referred to as an InGaAs layer) with large lattice constant as a light receiving layer, and positive and negative electrodes are arranged on upper and lower surfaces thereof Many substances surrounding us, including organic substances, water, gas and the like absorb waves having the wavelength in the near-infrared range of 1.7 μm to 2.6 μm, and therefore, such a sensor is expected to have wide applications in the field of environmental inspection, medical inspection, food analysis, temperature detection and the like.
In the near-infrared sensor described above, on an InP substrate, an InGaAs light receiving layer having a lattice constant larger than that of the InP substrate is formed. Therefore, the difference in lattice constant is adjusted by providing, between the InP substrate and the InGaAs light receiving layer, a plurality of step layers and a buffer layer formed thereon to be in contact with InGaAs layer. An epitaxial wafer having a lattice constant different from that of the substrate is referred to as a lattice mismatch type epitaxial wafer. The structure in which the step layers and a buffer layer are inserted between the substrate and the light receiving layer described above has been proposed repeatedly (for example, in Japanese Patent Laying-Open Nos. 2001-156324, 2001-102620 and 06-216403), since it has been found that the InGaAs layer has sensitivity in the near-infrared range.